1. Field of the Invention
This invention relates to a sensor system for measuring the light absorption in a target, e.g. a gas, comprising a first and a second optical transducer, one of which being a light source and the other being a light receiver, adapted to emit and detect, respectively, light within a chosen range of wavelengths, a partially reflective beam splitter adapted to reflect light to or from said first transducer, and to transmit light to or from said second transducer, and a lens or a group of lenses with a chosen focal length transmitting light to or from said transducers.
2. Description of the Related Art
Finding contents of a gas mixture is often done by transmitting a infrared beam with a known spectrum over a chosen path to a receiver. The receiver and instruments connected thereto are adapted to identify the wavelengths being absorbed by the gas mixture, and thereby to identify the elements present in the mixture.
The method is especially interesting when monitoring the atmosphere in e.g. a factory hall as a warning system for poisonous gases, explosion danger etc.
Usually the transmitter and receiver are separate units, and the transmitted light beam is aimed at the receiver. It is, however, difficult to obtain good mechanical stability in combination with angular adjustments with high precision.
To avoid the use of cables between the source and the receiver, and thus potential electrical noise, the measuring beam is also used to synchronize the receiver. This, however, results in a high probability for external disturbances in the measuring path, such as sunlight and blocking of the path.
This solution does not give any possibilities for monitoring the spectral distribution of the emitted radiation, and thus variations in this may also give 35 erroneous measurements. Often such instruments are connected to systems which close down an industrial process or oil/gas-production at a gas alarm. Production stops related to false alarms are expensive and reduces the operators confidence in the system. Therefore it is equally important that such instruments do not generate false alarms, as it is that all gas leaks are detected.
Another known type of gas sensors comprise the transmitter and the receiver in the same housing. The light beams is sent via a retro-reflector back to the receiver. Because of the requirement for high contrast in the received signal and the large amplitude of the transmitted beam compared to the received beam, the transmitted and the received beams have to be completely separated. This is normally done by using separate lens systems for the two transducers type, and keeping the transducers in separate compartments in the housing. This, however, is a complicated and voluminous solution, making the instrument large and expensive.
The instrument according to this invention is based on the solution with both transducers, the transmitter and the receiver, in the same housing. The transducers use the same collimating lens system directing the emitted beam towards a retro reflector. Thus all optical components may be integrated in the same compact unit, thus being adapted to use in a compact and robust instrument. Two sources, e.g. one generating the radiation used in the measurements and the other emitting a reference wavelength, may be integrated in the instrument. Also it is possible to measure the radiation before it is emitted into the beam path. This way an instrument may be constructed which does not produce erroneous measurements because of disturbances in the path or changes in the source.
A similar system is described in GB 2.219.656, comprising a transmitter and a receiver, both being directed towards a lens through a beam splitter. The retro reflector simplifies the alignment of the optical system, but the described system does, however have a disadvantage in that light transmitted from the transmitter will be reflected internally in the instrument, and affect the receiver.
Since the received radiation is in the range of two to three decades weaker than the emitted radiation it is impossible to reduce the internal reflections sufficiently to obtain acceptable results. To avoid this the sensor system according to the invention comprises a beam splitter including a first reflecting part and a second transmitting part being different from said first part, and a shield being positioned between said transmitting part and said first transducer.